FreeDV is a Digital Voice mode for HF radio.

You can run FreeDV using a free GUI application for Windows, Linux and
OSX that allows any SSB radio to be used for low bit rate digital
voice. Alternatively you can buy a SM1000 FreeDV adaptor that allows you
to run FreeDV on any HF radio without a PC or sound card.

Speech is compressed down to 700-1600 bit/s then modulated onto a
.25 kHz wide signal comprised of 16 QPSK carriers which is sent to the
Mic input of a SSB radio. The signal is received by an SSB radio, then
demodulated and decoded by FreeDV. FreeDV 700C is approaching SSB in
its low SNR performance. At high SNRs FreeDV 1600 sounds like FM,
with no annoying analog HF radio noise.

FreeDV was built by an international team of Radio Amateurs working
together on coding, design, user interface and testing.
FreeDV is open source software, released under the GNU Public License
version 2.1. The modems and Codec 2 speech codec used in FreeDV are
also open source.